The graphic images used in computer-generated displays have been stored in image memories at address locations mapping respective points at regular intervals along the raster scanning of a display image space. Each addressed location in image memory has contained a digital word, at least a portion of which has encoded the brightness, hue and saturation of a color pixel at the corresponding point in image space (and, in run-length encoding schemes, the value of succeeding pixels). A number of different schemes for encoding the brightness, hue and saturation of color pixels exist in the prior art.
One may analyze each color pixel as the sum of the three additive primary colors, red, green and blue, for example. The amplitudes of the red, green and blue components may each be coded in a number n of bits, n normally being in the range five to eight inclusive. Coding may be linear, logarithmic, or in accordance with some other function. It is also known to linearly encode red, green and blue in different numbers p, q and r of bits depending on their relative contributions to luminance. Encoding green in seven bits, red in five bits and blue in four bits is an example of such coding. The reader is referred to M. F. Cowlishaw's paper "Fundamental Requirements for Picture Presentation" appearing on pages 101-107 of PROCEEDINGS OF THE SID, Vol. 26/2, 1985, for a comprehensive treatment of coding additive primary colors in differing numbers of bits.
One may analyze each color pixel as the sum of a luminance-only primary color and two chrominance-only primary colors. The luminance-only primary represents whiteness or brightness of the pixel. The chrominance-only primaries do not correspond with any real color, but together are representative of the difference of any real color from the luminance-only primary. So the number of bits in these chrominance-only primaries differ little from the number of bits in the luminance-only primary, in order to avoid quantization errors in the summation of the primaries giving rise to posterization in the display.
One may arbitrarily code color values as addresses for memories, referred to as a color map memories. The memories respond to these addresses to supply, as read output, drive signals to the color display device that cause the desired color to be displayed. The memories, though operated as a read-only memories, may have provisions for changing the color maps they store. To facilitate changing the color maps these memories may be electrically-erasable programmable read-only memories or they may be random-access memories.
It is sought in a small computer system to provide a high degree of interchangeability among these various modes of image handling, so much so that composite displays comprising both computer-generated and camera-originated images as components can be created. A problem encountered in attempting to make such an image display processor is that pixels n camera-originated images of high quality are described by codes up to twenty-four bits long, which pixel codes are substantially longer codes in terms of bits than those normally used to describe the pixels of a graphics image in a computer-generated display.